We first applied transponders too high speed lines in 1974. Transponders provided the track-to-train communication medium required by a speed advisory system for the Advanced Passenger Trains. By using a tilting system, these trains were able to traverse curves at speeds much higher than is acceptable for conventional trains. So that drivers were not required to learn a new set of line speeds, each train was fitted with a speed advisory system, which displayed the appropriate line speed in the cab. Speed and other control information was encoded into pre-determined messages which were held in transponders mounted on the track; the transponders were activated from the train and the messages were read by train mounted interrogators.
More recently, journey times have been reduced by the maximum line speed on relatively straight track from 160 km/h to 200 km/h, and by the introduction of high performance non tilting rolling stock, notably the diesel powered High Speed Train (HST).
On the British railway system, high speed trains are manually driven, and the drivers are highly dependent on route knowledge. The only in-cab indications are provided by an automatic warning system (AWS) which advises of approaching restrictive signals and critical speed restrictions. Trains having a maximum speed greater than 160 km/h have two drivers in the cab; otherwise, driving cabs are generally single manned.
200 km/h is regarded as the maximum safe speed for driving to lineside signals; above this speed cab signalling and speed supervision are required. Even at lower speeds, some form of driver aid may be justified by reduced driver route learning, achievement of single manning on all trains, and improved safety.
The potential of transponder technology has therefore justified investment in a research programmed. The aim has been to build upon the experience and understanding derived from the earlier system, such that an appropriate transponder system can be developed quickly and confidently when a specific requirement arises.
One technique used in transponder technology is based on inductive coupling. Induction provides a relatively efficient mechanism for energising transponders, and ensures that they can be activated only when they are close to an interrogator. The latter is an important feature in a train control system.
The transponders have been designed to work in the low radio frequency band. This band is particularly suitable as, at these frequencies, the signals are completely unaffected by water and wet snow.
The transponders have had no internal power source and have received their energy from a carrier signal generated by the interrogator. The interrogator transmitter circulated a high frequency current in a loop antenna. The resulting magnetic field induced a corresponding current in a loop antenna in the transponder. The transponder generated a phase modulated return signal at half the frequency of the received carrier.
Analysis of the operation of the known transponder system has shown that a greatly enhanced return signal can be achieved by making the frequency of the return signal the same as that of the carrier. In the limit, all of the current flowing in the transponder antenna constitutes a source of the return signal. The return signal is seen at the interrogator as amplitude modulation of the transmitted signal.
Based on this concept the object of the invention is to provide a transponder system which has high energy efficiency and therefore enables a high modulation rate to be applied, i.e. a high data rate. This in turn allows short compact interrogator aerials to be used.
According to one aspect of the invention a transponder circuit for transmitting a coded message comprises a tuned circuit tuned to a first, resonant frequency and detunable to a second frequency by an associated modulator, whereby the coded message from said transponder can be made up of sequencies of said frequencies in predetermined combinations.
According to another aspect of the invention there is provided a loop for inductively coupling with an interrogator to transmit a binary coded message to the interrogator, wherein the loop forms part of a tuned circuit which is tuned to a first, resonant frequency and which is detunable by an associated modulator to a second, lower frequency whereby a binary code can be created utilizing said two frequencies.